Motivation

I like her being a barista because:

It fits with the increasingly popular Cozy drinks vibe popularized by Starbucks.
It provides Adelle a social outlet.
1. Which also provides opportunity for her to interact with more plot characters.
It is a good opportunity for Condescension.
It has some potential relationship, either directly or subconsciously, to Adelle as a maid.
It’s unusual for this sort of genre.
Unlike some jobs which can be at least partially performed from home, she will need to commute through the dangerous city to her job location.
It turns the potential location of a Tea shop into an anchor point (by having a major character working there. The first book had a Tea shop which featured in many scenes, but it was still mostly background. Adelle working there gives it more distinction and identity.)

Boss